Recombination vs. Recombinants

[dreamtim]

I've read a phrase that says: 'because only two of four chormatides are involved in crossing over, the result of one recombination case is two recombinants gametes and two parental gametes, therefore, the rate of the recombinants is half the rate of the recombination

I couldn't understand what the meant in red.

[Cat]

1. You have a tetrad in crossover (say AB maternal and CD paternal):

------------A---------/---------B---------
------------A---------/---------B---------
------------C---------/---------D---------
------------C---------/---------D---------

Crossover results in:

------------A---------/---------B---------
------------C--x------/---------B---------(x designates crossover point)
------------A--x------/---------D---------
------------C---------/---------D---------

Now, segregation of alleles into 4 haploid daughter cells gives you 2 recombinant and 2 paternal gametes. If recombination event occurs in 1 out of 10 cells (0.1), than if you start with 10 cells, that will produce 40 gametes out of which (9 cells were non-recombinant) = 9 x 4 = 36 paternal gametes + 1 cell where recombination occurred giving you 2 recombinant and 2 paternal gametes. Thus, you have 2 recombinant : 38 paternal gametes and rate of recombinants is 2/40= 0.05 (which is 1/2 of the rate of recombination of 0.1).